Electrochemical Machining (ECM) Questions

• Electrochemical Machining (ECM) is presented by Dr. P. Saha from the Department of Mechanical Engineering at IIT Kharagpur.

Basic Electro-chemical Cell

• The basic electrochemical cell involves the movement of electrons between an anode and a cathode, facilitated by an electrolyte solution containing metal ions with positive charge of x (M+X).

Electro Chemical Processing Types

• Surface Treatments
• Coating, which includes thin coating (electroplating) and thick coating (electroforming).
• Machining: electrochemical machining.

Basic Electrochemistry of a Plating Bath

• Anode reactions: Metal (M) releases electrons to become metal ions (Mn+), which can react with other ions in the electrolyte.
• Electrolyte (ionization): Electrolytes like MmZn dissolve into metal ions (Mn+) and other ions (Zm-), and water ionizes into H+ and (OH)-.
• Cathode reactions: Metal ions gain electrons to become metal deposits, and hydrogen ions gain electrons to form hydrogen gas.

Concept of Electrochemical Machining

• ECM is the reverse of electroplating.
• The workpiece (job) becomes the anode and erodes.
• The tool remains unaltered, experiencing neither deposition nor erosion.

Basic Electrochemistry in Electrochemical Machining

• Anode: Iron (Fe) turns into Fe2+ + and releases electrons, which then reacts to form FeCl2 and Fe(OH)2.
• Electrolyte: NaCl dissociates into Na+ and Cl-, and H2O into H+ and (OH)-.
• Outcome: There's no tool damage because there is neither deposition nor erosion.

Basic Scheme of an ECM Machine

• The setup includes a DC power supply, an electrolyte tank, a pump, and a fixture.
• Electrolyte and sludge are separated using a centrifuge.

Schematic Diagram of an ECM Machine

• The machine includes a power supply, voltmeter, ammeter, electrolyte, filtration system, and a pump.

Voltage Drops in ECM

• There are several voltage drop zones.
• Helmholtz Double Layer is ~ 10-7mm
• Anode Over Voltage, where anode drop = ~ 1.5V
• Ohmic Drop, the ELECTROLYTE = I2 R
• Applied voltage i.e CELL VOLTAGE = 5 - 50V
• Continuous or PULSED CURRENT DENSITY = 10 to 100 A/cm²
• Cathode Over Voltage, cathode drop = ~ 1.5V
• The Gap distance is ~ 10-7mm

Faraday's Laws of Electrolysis

• The amount of substance deposited or dissolved is proportional to the quantity of electricity passed through the electrolyte (W ∝ Q, Q = I.t).
• The quantities of substances liberated or deposited are proportional to their electrochemical equivalent weights (ECE); (W ∝ ECE, ECE = A/v). 'A' representing atomic weight and 'ν' representing valency.

ECM Formulas:

• The substance weight W equals (1/F)(A/v)Q or (1/F)(A/v) I.t, F=96,500 Coulombs.
• The material removal weight is: Ŵ = (W / t) = (A/F.v) I [gm-equivalent /s].

Material Removal Rate (MRR)

• MRR = (A/F.v)(I/ρ) , measured in [cm³/s]
• Feed rate = (A/F.v)(Jc/ρ) measured in [cm/s], with Jc as the current density = I / Surface area [A/cm²]. For an ALLOY, with 'n' number of elements present in it; Valency, atomic weight and their percentage presence are represented by v₁ to vₙ, A₁ to Aₙ,and X₁ to Xₙ respectively; So the removal rate, W = (100.I / F)[1/∑X₁v₁/A] [gm-equivalent /s] So material removal rate at any point is proportional to current density.

ECM Profile Transfer

• The transfer of the tool profile to the workpiece is possible in ECM.

ECM Dissolution Reactions

• For dissolution reactions to proceed in ECM, a constant voltage is applied to the electrodes with zero feed of the cathode tool,the specific resistance of the electrolyte is important.

Rate of Anodic Dissolution

• Determined by dW = a. dh.p = (1/F) (ECE) I dt. Rate of gap change: (dh/dt) = (1/F) (ECE) (I / aρ).

• The initial gap ho increases to h₁ within time, t: h₁2 – ho² = 2Ct .The machining rate decreases accordingly, and finally ceases.

• Where With Feed=S, removal rate is written (dh/dt) = C /h-s. For steady state (MRR) (dh/dt)=0: (10) (dh'/dt') = (1- h') / h'. t = [ (h｡´- h₁´) + In {( h｡´- 1)/ (h₁΄- 1)}]

Equilibrium Factors for Gap

• The electrode gap takes an EQUILIBRIUM GAP, regardless of initial gap.

Critical ECM Features

• The process is self-regulated, maintaining a constant gap, depending on the initial gap and feed rate.
• The value of equilibrium depends on feed rate.
• Higher feed rates yield smaller gaps, higher current density, and improved job accuracy.
• Surface finish and material hardness are independent factors

Key Aspects of ECM

• No tool wear occurs.
• Higher material removal rates are achieved with higher current, if inter-electrode gap condition is maintained.
• Electrolyte flow avoids ion concentration, tool deposition, and electrolyte overheating.

Electrolyte Concerns

• Is the electrolyte flow is restricted by design maximum, the permissible current can be calculated for MRR max.
• Then, permissible feed rate and gap is determinable

ECM Parametric Values

• Direct Current Voltage: 5 to 30 V (continuous or pulsed source). Current: 50 to 40,000 A. Current Density ranges from 10 to 500 A/cm².
• Electrolyte: NaCl (60 to 240 g/l), NaNO3 (120 to 480 g/l), Proprietary Mixture. Temperature: 20 to 50° C.
• Fluid speed is 1500 to 3000 m/minute

Tool and Gap Examples for ECM

• Typical tools for ECM.
• An ECM Die Sinking Machine with Power supply.

Working Parameters

• Frontal Working Gap: 0.05 to 0.3 mm
• Feed rate: 0.1 to 20 mm/min
• Electrode material: Brass, Copper, Bronze
• Surface roughness: 0.1 to 2.5 um

Main characteristics of ECM

• MRR in ECM does not depend on mechanical properties of the metal, depends on work piece composition.
• Accuracy of ECM depends on the dimension of the cutting work piece and is approximately 0.05 mm to 0.3 mm using continues current, 0.02mm- 0.05 mm using pulsed.
• ECM has residual stress and energy consumption.

ECM Tool Design Aspects

• Design of ECM tools considers electrolyte flow (flow path with a corner radius of 0.7, to 0.8 mm), insulation and strength.

Electrolyte Heating Factors

• Temperature increases the resistivity valve of the electrode in the direction of the flow path

ECM Gas Buildup Factors

• Electrolyte resistivity in the direction of flow increases as gas concetration builds up.

ECM Sludge Buildup Factors

• Variation of sludge concentration

ECM Considerations

• Maching depends on if two surface are inclined at angles proportional to the length 's', feed rate

Issues

• As ECM is an molecule-by- molecule dissolution process, can we get a nano-level surface finish through ECM?

Special Applications

• Additional applications of this process include electrochemical cutting, milling, turning, and shaping a form.

ECM-specific Deburring

• Electrochemical deburring offers simple operation, often uses a static electrode, enables gradual smoothing and is utilized widely across the industrial spectrum.
• Different materials need different formulas.

Hole Drill with ECM

• Electrochemical hole drilling for small holes can be done by conventional electrolyte tubes (0.127 mm < Hole diameter < 0.890 mm), or electro-stream drilling, Application – small cooling hole in super alloys. Negatively charged acid electrolyte stream with high velocity
• Small hole production techniques encompass Electrostream drilling, shaped tube electrolyte machining (STEM)..

Electro-stream Drilling

• Dwell Drilling: Shallow and less accurate holes. Limited depth of hols, nozzle movement is available. .
• ES Drilling Applications, ES drilling is used for high-depth production. Burr-free Absence of recast and metallurgical defects

These questions cover several aspects of Electrochemical Machining (ECM). It includes the rate of change of the gap, the effect of feed rate, and the advantages of using ECM for hard materials. It also touches on the role of the non-dimensional equation.